The specific aim of this project is to evaluate the nature of communication skills in patients with Rett Syndrome. Our focus is two-fold: to investigate 1) the basic nature of the disordered communication and 2) how developmental and intervention-related changes affect communication skills. We propose to carry out behavioral assessment of speech and language skills in an effort to delineate the fundamental nature of disordered communication and the status of residual function. In addition, we propose to carry out electrophysiologic and electroacoustic evaluation of the auditory system to quantify the extent of sensory and perceptual involvement as contributing factors to communication disorder. Experimental subjects will be 70 patients with a diagnosis of Rett Syndrome and 12 control patients with a diagnosis of autism. All subjects will receive a complete speech and language evaluation and a complete audiological evaluation, including behavioral audiometry, immittance audiometry, and evoked potential audiometry. In addition, a smaller group of patients (N=24) will be evaluated as part of a drug treatment regimen. Initial, baseline evaluation will be followed by electrophysiologic evaluation during an acute treatment stage. A double-blind, crossover design will be used for the chronic treatment stage. Full evaluation will be carried out after four months of treatment with one drug, after a one month "wash-out" period, and, again, after four months of treatment with another drug. Longitudinal studies will be carried out on a yearly basis over a five-year period on 30 patients. Additionally, all subjects who pass from one classification stage to another will be evaluated following the classification change. The significance of this project lies in its contribution to the understanding of disorder communication related to Rett Syndrome. We may be able to define precise behavioral, electroacoustic, and electrophysiologic patterns that characterize development of the disorder and that provide evidence for benefit from pharmacologic intervention.